The coefficient of earth pressure at rest, Ksub.0, is a significant mechanical parameter, and the investigation of the Ksub.0 of coarse-grained soil has important theoretical significance and applicational value in geotechnical engineering. However, there are few studies on the influence of the initial dry density (ρd) on the Ksub.0 of coarse-grained soil due to the limitations of the related test instruments or methods. A series of Ksub.0 tests for two types of coarse-grained soils (rockfill soil and sandy-gravel soil) were conducted based on a newly developed large-scale apparatus to reveal the relationship between the Ksub.0 and ρd of coarse-grained soil. The test results showed that the Ksub.0 of coarse-grained soil decreases with an increase in vertical stress, and this trend tends to be gentle with respect to the increase in vertical stress. The results also implied that there was a negative linear relationship between the Ksub.0 of coarse-grained soil and ρd. Furthermore, a comparative analysis between rockfill soil and sandy-gravel soil indicated that the relative equation proposed by the authors was appropriate for any type of coarse-grained soil with any ρd. Moreover, an empirical equation that can accurately describe the effective relationship of σv and ρd with the Ksub.0 for coarse-grained soil was proposed, and the accuracy of the empirical equations were verified by the Ksub.0 test results concerning sand-gravel soil. Finally, based on the published research findings, the empirical equations' applicability to any coarse-grained soil was verified.